In recent years, optical modulators that modulate light in response to RF signals have become one of the essential components in configuring optical fiber communication networks. In particular, a Mach Zehnder interferometer type optical modulator (or short “Mach Zehnder modulator (MZM)”) that uses a waveguide-type optical interferometer enables high-speed modulation of 40 Gbps or higher. However, as the bit rates in optical communication networks increase optical loss in optoelectronic network components, such as Mach Zehnder modulators, becomes more of an issue. To overcome this problem there have been attempts to integrate semiconductor optical amplifiers (SOAs), fixed wavelength lasers, tuneable lasers and other active sections into MZM devices.
To minimize device length and maximize speed in the phase modulating (PM) core of an MZM the multiple quantum well (MQW) core typically needs a large number of quantum wells, for instance, 20 to 30 or more. This results in a very tightly confined optical mode, which, in turn, leads to a very large angular divergence (with a typical FWHM of the far field being larger than 50 degrees). Such a large angular divergence and small mode size result in very poor coupling efficiency between an MZM and an optical fiber, as well as very tight alignment tolerances.
To overcome this issue, a lot of conventional devices incorporate a waveguide mode transformer on the input and output facet of the MZM. The waveguide mode transformer is generally realized by selective area epitaxy to butt couple the passive waveguide mode transformer to the MZM core, because selective area epitaxy allows both the thickness and band gap of the regrown material to be varied as a function of length by the use of a patterned oxide mask. However, such an additional growth step adds fabrication complexity and can reduce device yield resulting in an increase in device costs.
Moreover, the tightly confined optical mode of the MZM core is also not well matched to that required for other active optical elements including SOAs and laser gain regions that need to be monolithically integrated with the MZM. Typical SOA or laser gain regions require about 3 to 6 quantum wells and operate at a much lower optical confinement factor of between 20 to 30% compared to around 70% for the MZM phase modulating core.
Again to overcome this difference in optimum design a mode transformer or expander is typically required between the SOA or laser active and phase modulating region of the device, to modify the mode size without introducing excessive loss and/or reflection. Again this has traditionally been done using selective area epitaxy.
Since the inclusion of an SOA and or laser gain region already requires the use of an additional stage of regrowth to grow the gain regions the addition of yet another step to add the mode transformer again has the potential to reduce yield.
There is, therefore, a need for an improved semiconductor optical apparatus, in particular an improved semiconductor optical apparatus allowing to simplify mode expansion at the facet of a Mach Zehnder Modulator (MZM) and to simplify the integration of an MZM core with forward biased active sections such as semiconductor optical amplifiers (SOAs) or semiconductor lasers.